Dissecting the cellular aging process

The majority of prevalent non-infectious diseases are associated with age, yet the mechanism by which these diseases dramatically increase with age is unclear. A number of intellectually attractive hypotheses to explain age-associated decline have been generated by studies in model organisms. While there is a general consensus that accumulation of cellular damage is the basis for this decline, a molecular mechanism for what actually causes aging in any organism remains elusive.

Our overarching goal is to identify the molecular changes that cause aging, as well as the downstream age-associated events that lead to cellular decline. The budding yeast Saccharomyces cerevisiae is a great model system for studying the aging process in eukaryotic cells. With each asymmetric cell division of this budding yeast, the vitality of a “mother” cell declines, and its limited replicative life span has been used as a model to identify and study conserved genetic and environmental processes that contribute to aging in metazoa. The use of budding yeast to study aging has not been fully exploited because of the difficulty in isolating replicatively aged cells. We recently overcame this limitation by developing a technique called the Mother Enrichment Program (MEP – Lindstrom & Gottschling, 2009) that allows us to isolate and examine large populations of synchronously aged cells.

Viewing aging through the lens of interconnectivity

Our approach to studying aging considers the process in light of a fundamental property of biological systems – interconnectivity. All levels of interaction contribute to the ultimate phenotype of an organism: interactions between tissues, cells, organelles, metabolic pathways, genes, and individual molecules. With the help of network analysis, the complexity of such interactions can be visualized to develop new ideas and hypotheses about the aging process. For instance, if we examine a network of interactions within a cell and consider each organelle as a subsystem within the network, then what happens to the other connected organelles when one becomes dysfunctional with age? If a subsystem decays with age and does indeed affect a connected organelle, which interactions are required for this to occur? This simple idea focuses upon connections that create interdependency between two subsystems – e.g. one organelle produces a molecule required for the proper function of another organelle. However, in considering a network of interactions it is likely that more than one subsystem is sensitive to aging, possibly through distinct routes. Dissecting how these types of events occur is critical to developing a better understanding of aging.

We also consider a second type of interaction in the aging process: the response of a biological system to change (i.e. the basis of homeostasis and biological anticipation). As an organism ages, what response or compensation occurs in a biological network? What is the consequence of the response? For instance, is compensation “successful” such that it helps to prevent dysfunction, or does compensation result in unintended negative consequences, and throw different, interconnected, subsystems into dysfunction?

How do cellular subsystems break down?

Taking advantage of the MEP, we have begun to address these questions at the cellular level. To this end we have identified several cell biological subsystems that experience an age-associated decline. For example, we identified an age-associated mitochondrial change that is conserved throughout eukaryotes. We discovered a series of causal events that lead to this change and also impact overall life span of the cell. The earliest step in the aging process that we have defined so far is a reduction in vacuole acidity with age, which in turn leads to a loss of mitochondrial membrane potential (Hughes & Gottschling, 2012). The inability of the vacuole to store amino acids when vacuolar pH increases is responsible for the loss of mitochondrial membrane potential, though the molecular details of this link remain an area of investigation. Loss of mitochondrial membrane potential in turn causes dysfunction

of a number of mitochondrial processes. One of note is reduced biosynthesis of iron-sulfur complexes (ISC), an essential cofactor in a number of enzymes, including those involved in DNA replication and repair. In fact, we find that reduced ISC levels during mitochondrial dysfunction lead to nuclear genome instability – a relatively late age-associated phenotype (Veatch et al. 2009, McMurray & Gottschling, 2003).

By approaching the study of aging through the lens of interconnectivity, we have identified several linked causal events in the aging process. Of course, many of the questions posed above remain unanswered. But given our successes so far, we will continue to address them using the combination of approaches afforded by this wonderful model system.